This invention relates to magnetic resonance imaging ("MRI") magnet and probe design for imaging a region external to probe for biomedical or industrial imaging applications. More specifically, a primary magnet provides a homogeneous field external to the surface of the magnet suitable for imaging. Preferably, the primary magnet is a cylindrical permanent magnet, but can be a symmetrical magnet having rectangular shape or having an outer surface defined by a surface of revolution. A MRI catheter containing such a magnet and a rf coil is particularly suited for endoscopical imaging of tissue of the artery wall, rectum, urinal tract, intestine, esophagus, nasal passages, vagina and other biomedical applications. It also is suitable for geophysical, oceanological and industrial applications involving MRI imaging or spectroscopy of regions external to the magnet.
Solenoidal MRI magnets (superconductive, resistive) as well as iron core C and E shape electromagnets or permanent magnets are known for imaging of the whole body and its extremities. However, such whole body MRI magnets are very expensive and bulky. They are not particularly portable and, thus, are not generally widely suitable for endoscopical imaging of various parts of body.
Typically, MRI magnets are designed to provide a homogeneous magnetic field in an internal region within the magnet, i.e., in a large central bore of a solenoid or the air gap between the magnetic poles. A patient or object to be imaged is usually positioned in the homogeneous field region located in the central air gap for imaging. In addition to the main or primary magnet that provides the background magnetic field B.sub.o, the MRI system typically has gradient and rf coils which are used for spatial encoding and exciting the nuclei for imaging. These gradient field and rf coils are typically located external to the patient inside the primary magnet surrounding the central air gap.
To provide a higher resolution of artery wall images, Kandarpa et al. investigated the feasibility of a miniature endoluminal magnetic resonance (MR) detection coil (i.e., rf receiving coil) for imaging mural and perimural anatomy of small tubular structures. J. Vascular and Interventional Radiology 1993, 4:419-27. The authors concluded that, with further development, such a detection coil may be useful for studying atherosclerosis and for providing imaging guidance during endoluminal MR interventions. In vivo tracking and accurate placement of catheters equipped with miniature rf coils is described in AJR 1995, 164:1265-70. However, use of catheters equipped with miniature rf coils as described in these publications still requires positioning the patient in conventional large specialized MRI magnets. This environment can result in deficient images because the various orientations of the rf coil, e.g., in an artery, will not be positioned always colinearly with the rf excitation field. Also, very large gradients are needed with external .o slashed. gradients to get sufficient spatial resolution.
Contrast angiography is the conventional technique used for vascular imaging. However, this technique produces a lumenogram (or two dimensional projection) rather than a true three dimensional image. This technique requires exposure to ionized radiation. To address some of the limitations of angiography, ultrasound has also been developed. Intravascular ultrasound has been successful in defining the location of calcifications and directing atherectomy, however, it has not been clinically useful in tissue characterization of atherosclerotic plaque and arterial obstruction. Angioscopy has provided valuable contributions to understanding unstable angina but it is limited by its ability to view only the surface of an atheroma in a bloodless field. External magnetic resonance angiography has not achieved sufficient resolution to be useful for planning or assessment of vascular interventions.
U.S. Pat. No. 4,350,955 describes means for producing a homogeneous magnetic field remote from the source of the field, wherein two equal field sources are arranged axially so that their fields oppose, producing a region near the plane perpendicular to the axis midway between the sources where the radial component of the field goes through a maximum. A region of relative radial field B.sub.r homogeneity may be found near the maximum. See also, J. Mag. Resonance 1980, 41:400-5; J. Mag. Resonance 1980, 41:406-10; J. Mag. Resonance 1980, 41:411-21. Thus, two coils having opposing polarity are positioned axially in a spaced relationship to produce a relatively homogeneous toroidal magnet field region in a plane between the magnets and perpendicular to the axis of cylindrical symmetry.
Thus, it is desirable to have new and better devices and techniques for biomedical MRI applications such as intravascular imaging and tissue characterization. Such devices and techniques could be applied to a wide variety of imaging and tissue characterization uses such as, for example, transurethral or transrectal imaging of the prostrate, imaging of the esophagus, colon, and small intestine, etc. The design of a small probe to image a target volume external to its surface can enable high resolution images in the imaging volume.